How to find army records for the South African (Boer) War 1899-1902
(a very brief summary just to get you started)

For those who died:
Search the Index to Natal and South Africa Forces deaths 1899-1902. It is available at the Family Records centre (FRC) in London and The National Archives (TNA) at Kew. The index has been copied onto fiches and can be viewed at many Local Studies' Libraries and County Record Offices.

For Officers' services:
Search the 'Army List' to trace details of an officer's career. Published annually since 1754, you can find partial runs of the list in most large libraries.

For Other ranks:
Search the Medal Rolls at TNA in document class WO100
Most army records are held in TNA - for service in the army see the Soldiers' Document in document class WO97.

Those who served in the Imperial Yeomanry are in class WO128. A publication (available at TNA) is useful here:

'The roll of the Imperial Yeomanry, Scottish Horse and Lovats Scouts, 2nd Boer War 1899-1902' by K J Asplin - it is an 'alphabetical list of 39,800 men of these volunteer forces who enlisted for the 2nd Boer War.'

A selection of records relating to disabled Naval ratings and soldiers who received disability pensions and who served between 1877 and 1914 are in a file PIN71 at TNA. This is searchable using 'The Catalogue' the search engine for TNA records available at
https://www.catalogue.nationalarchives.gov.uk/default.asp